This program considers the roles of complexity and of variability in sound error patterns and their possible interaction in the treatment of children with speech disorders. Complexity is appealed to as a universal property of language, defined here in terms of markedness. Based on previous treatment efficacy research, it is predicted that greater improvements will occur following treatment on more complex aspects of the sound system than on less complex sounds. The first goal of this project is to further evaluate the role of complexity in effecting change in children's sound systems. Variability is appealed to in terms of the number, extent, and type of errors that a child produces for a given target English sound. Based on previous research, it is predicted that greater gains will result from treatment on sounds that are produced more variably than on those produced less variably. Because no previous research has experimentally evaluated this prediction, the second goal of this project is to consider the role of variability in promoting change in children's sound systems. As of yet, no research has evaluated how complexity interacts with other variables for treatment sound selection. Because sounds that are complex can also be produced variably or non-variably, the third goal of this project is to experimentally evaluate the interaction between complexity and variability by determining the combined effects of the two variables on children's sound systems. Eighteen monolingual English-speaking children with speech disorders will participate, with each assigned to 2 combined treatment conditions based on variability and complexity. These conditions will be evaluated separately and in conjunction with one another. The results of this project will provide guidelines for effective treatment of speech disorders, and they will help in determining the additive (or precedence) relationships between different types of variability and relative complexity. Theoretically, the results will aid in establishing specific aspects of sound systems that contribute most to speech sound development.